FTIR spectroscopic investigations of supersaturated NaClO₄ aerosols

Supersaturated NaClO₄ aerosols have been studied using a Fourier transform infrared (FTIR) spectrometer coupled with an aerosol flow tube (AFT). Compared with previous Raman results, the water O-H stretching envelope in the supersaturated solutions of NaClO₄ aerosols was more structured in response to changing RH, revealing at the same time the existence of water monomers weakly hydrogen-bonded with ClO₄^- at extremely high concentrations. Due to enhanced ion interactions in the supersaturated solutions of NaClO₄ aerosols, the formation of contact ion pairs (CIPs) could be observed without component decomposition for the nondegenerate ν₁ band of ClO₄^-, and the degenerate ν₃ band of ClO₄^- was successfully related to the formation of CIPs in NaClO₄ solutions. Based on these observations, a new mechanism featured by the attack of ClO₄ upon hydrated Na⁺ for CIPs formation in the supersaturated solutions of NaClO₄ aerosols was further proposed. The anhydrous NaClO ₄, characterized by the upper limit deliquescence relative humidity (DRH) of ∼43% and the disappearance of the ν₁ band of ClO ₄^- in the infrared spectra, was observed to form on the silicon windows at low RHs.